Meiosis is a specialized cell division used by sexually reproducing organisms to produce haploid gametes from diploid cells. This reduction in chromosome number is accomplished by having two rounds of chromosome segregation follow a single round of chromosome duplication. Failures in meiotic chromosome segregation in humans lead to infertility and birth defects such as Trisomy 21, the leading cause of mental retardation in the United States. In addition, meiotic chromosome segregation requires the repair of programmed double strand breaks (DSBs). DSB repair in mitotically dividing cells is critical for genome integrity and the prevention of cancer. Budding yeast is an excellent model system for studying meiosis. Mice and yeast mutated for orthologous meiotic genes are phenotypically similar, demonstrating that studies of meiosis in yeast are likely to be illuminating with regard to meiosis in humans. The first meiotic division (MI) is unique in that sister chromatids segregate to the same pole (called reductional segregation). Proper MI segregation requires that homologous chromosomes be connected to each other by crossovers between non-sister chromatids in combination with sister chromatid cohesion. Recent work has shown that crossing over between non-sister chromatids during meiosis is promoted in part by active suppression of DSB repair between sister chromatids. This suppression is mediated by Mek1, a meiosis-specific serine/threonine kinase. Understanding the molecular mechanism by which Mek1 inhibits meiotic sister chromatid repair requires identification of the substrates phosphorylated by this kinase. We will use novel biochemical and genetic strategies to identify Mek1 targets and characterize phosphorylation- defective versions of these proteins elucidate this process. In addition we will test a specific model for how cohesins may be involved in suppressing meiotic intersister DSB repair. Repair of meiotic DSBs utilizes two recombinases for strand invasion: Dmc1, which is meiosis-specific and Rad51, the major recombinase in vegetative cells. How the action of these two strand exchange proteins is coordinated during meiotic recombination not yet understood. Recent biochemical experiments have suggested a novel mechanism for regulating Rad51 activity mediated by Mek1 phosphorylation of an accessory protein, Rad54. One goal is to determine whether this mechanism functions in meiotic cells. PUBLIC HEALTH RELEVANCE: Failures in the evolutionarily conserved cell division of meiosis result in infertility and birth defects such as Down syndrome. Proper meiotic chromosome segregation requires double strand break repair-a process also required to maintain genome integrity and prevent cancer. This grant is focused on understanding the molecular basis for proper chromosome behavior during meiosis-knowledge that may ultimately lead to the diagnosis and/or prevention of certain types of infertility, birth defects and even cancer.